For crystal growth of a GaN group material, a substrate that does not lattice match therewith, such as sapphire, SiC, Spinel, and recently Si and the like, has been used due to the absence of a substrate that lattice matches with GaN group materials. However, a GaN film produced contains dislocations of as many as 1010 points/cm2 due to the absence of lattice match. While high luminance Light Emitting Diodes, semiconductor lasers and the like have been realized in recent years, reduction of dislocation density has been desired for improved properties.
As a method for reducing the dislocation density when, for example, a GaN group semiconductor substrate and the like are grown on a buffer layer and a GaN substrate by vapor phase growth, a method comprising formation of a partial mask on the aforementioned substrate and selective growth to achieve crystal growth in the lateral direction has been proposed, thereby to give a high quality crystal having a reduced dislocation density (e.g., JP-A-10-312971).
It has been clarified that, according to the above-mentioned method, a problem occurs that, in the part of a mask layer where growth in the lateral direction occurred, the C axis tilts in a slight amount toward the direction of the lateral growth, which in turn causes a new problem of degraded crystal quality (Abstracts G3.1 of MRS 1998 Fall Meeting). This can be confirmed through measurement (φ scan) of the incident orientation dependency in X-ray rocking curve measurement (XRC). That is, a full width at half-maximum (FWHM) of X-ray rocking curve by incident X-ray from the direction of lateral growth is greater than the FWHM value by X-ray from a stripe direction of a mask layer, which means the presence of orientation dependency in the micro tilting of the C axis. This suggests a possibility of inducing a number of new defects in the junction part of the lateral growth on the mask.
As the mask layer material, SiO2 is generally used. However, a problem has been found that, when a crystal growth layer is laminated thereon, the Si component transfers into the crystal growth layer, constituting a problem of autodoping contamination.
When a semiconductor material containing Al, such as AlGaN, is grown on a substrate having an SiO2 mask layer, crystal growth occurs on the mask layer, too, preventing effective selective growth itself.
In an attempt to solve such problems, a method has been proposed wherein a stripe groove processing is applied to a substrate having a buffer layer and a GaN layer formed on an SiC base substrate, which groove reaching the SiC layer to form a convex, and crystal growth is started from the GaN layer on the top of this convex (Abstracts G3.38 of MRS 1998 Fall Meeting). According to this method, a selective growth without an SiO2 mask layer is possible, whereby resolving various problems caused by the use of the aforementioned SiO2 mask.
For the above-mentioned method, a sapphire substrate can be used as the base substrate and the method thereof is also disclosed (e.g., JP-A-11-191659). The above-mentioned method requires steps of crystal growing a buffer layer material and a GaN group material on a sapphire base substrate, taking the substrate out from a growth furnace to apply a groove processing, and then crystal growing again, thus posing a new inconvenience of complicated production process, increased number of steps, higher cost and the like.
In addition, a method for suppressing propagation of dislocation by forming concavo-convex grooves on a substrate and growing a gallium nitride group semiconductor while forming a cavity in the concave part has been disclosed (JP-A-2000-106455). According to this method, a low dislocation density area can be formed by a single growth, but a cavity needs to be formed. Thus, when a light emitting element and the like are prepared, it is inconvenient when releasing the heat generated in the light emitting part to the substrate side, thus problematically encouraging thermal degradation of laser diode and the like. Moreover, since this method does not actively control propagation of dislocation, the dislocation propagates to the upper part of the convex part, problematically making reduction of dislocation density insufficient.